This invention relates to drill bits, specifically drill bit assemblies for use in oil, gas and geothermal drilling. Various methods have been devised for passing a drill bit assembly through an existing cased borehole and permitting the drill bit assembly to drill a new portion of the borehole that is of a larger diameter than the inside diameter of the existing borehole. However, bi-center drill bits often experience bit whirl because of the harsh conditions as well as the lack of stability when drilling below the earth's surface.
The prior art has addressed issues dealing with the stabilization of drill bits, specifically bi-center drill bits. Such issues have been addressed in the U.S. Pat. No. 5,957,223 to Duster, which is herein incorporated by reference for all that it contains. The '223 patent discloses a method and apparatus for reaming or enlarging a borehole using a bi-center bit with a stability-enhanced design. The cutters on the pilot bit section of the bi-center bit are placed and oriented to generate a lateral force vector longitudinally offset from, but substantially radically aligned with, the much larger lateral force vector generated by the reamer bit section. These two aligned force vectors thus tend to press the bit in the same lateral direction (which moves relative to the borehole sidewall as the bit rotates) along its entire longitudinal extent so that a single circumferential area of the pilot bit section gage rides against the sidewall of the pilot borehole, resulting in a reduced tendency for the bit to cock or tilt with respect to the axis of the borehole. Further, the pilot bit section includes enhanced gage pad area to accommodate this highly-focused lateral loading, particularly that attributable to the dominant force vector generated by the reamer bit section, so that the pilot borehole remains in-gage and round in configuration, providing a consistent longitudinal axis for the reamer bit section to follow.
U.S. Pat. No. 5,979,577 to Fielder which is herein incorporated by reference for all that it contains, discloses a drilling tool operational with a rotational drive source for drilling in a subterranean formation where the tool comprises a body defining a face disposed about a longitudinal axis, a plurality of cutting elements fixedly disposed on and projecting from the tool face and spaced apart from one another, and one or more stabilizing elements disposed on the tool face and defining a beveled surface.
U.S. Pat. No. 6,227,312 to Eppink, et al. which is herein incorporated by reference for all that it contains, discloses a drilling assembly that includes an eccentric adjustable diameter blade stabilizer and has a housing with a fixed stabilizer blade and a pair of adjustable stabilizer blades. The adjustable stabilizer blades are housed within openings in the stabilizer housing and have inclined surfaces which engage ramps on the housing for cramming the blades radically upon their movement axially. The adjustable blades are operatively connected to an extender piston on one end for extending the blades and a return spring at the other end for contracting the blades. The eccentric stabilizer also includes one or more flow tubes through which drilling fluids pass that apply a differential pressure across the stabilizer housing to actuate the extender pistons to move the adjustable stabilizer blades axially upstream to their extended position. The eccentric stabilizer is mounted on a bi-center bit which has an eccentric reamer section and a pilot bit. In the contracted position, the areas of contact between the eccentric stabilizer and the borehole form a contact axis which is coincident with the pass through axis of the bi-center bit as the drilling assembly passes through the existing cased borehole. In the extended position, the extended adjustable stabilizer blades shift the contact axis such that the areas of contact between the eccentric stabilizer and the borehole form a contact axis which is coincident with the axis of the pilot bit so that the eccentric stabilizer stabilizes the pilot bit in the desired direction of drilling as the eccentric reamer section reams the new borehole.
U.S. Pat. No. 6,659,207 to Hoffmaster, et al. which is herein incorporated by reference for all that it contains, discloses a bi-center drill bit which includes a bit body having pilot blades and reaming blades distributed azimuthally around the body. The blades have cutting elements disposed thereon at selected positions. The body and blades define a longitudinal axis of the bit and a pass-through axis of the bit. In one aspect, selected ones of the pilot blades include thereon, longitudinally between the pilot blades and the reaming blades, a pilot hole conditioning section including gage faces. The gage faces define a diameter intermediate a pilot hole diameter and a pass-through diameter defined, respectively, by the pilot blades and the reaming blades.